Gas turbine engine components operating at very high temperatures typically rely on applied protective coatings such as platinum aluminde, etc. as a first line of defense against oxidation and sulfidation (i.e., hot corrosion). However, where the protective coating is worn off, eroded or otherwise breached, it is highly desirable that the exposed parent metal itself possess sufficient oxidation and/or sulfidation resistance for durability. It is known that the high-temperature oxidation resistance of various cast or wrought nickel or cobalt base superalloys can be significantly improved without metallurgical detriment, by avoiding brittle phases, for example, by retaining relatively small amounts of certain extremely reactive elements such as Lanthanum (La) or Yttrium (Y) (hereinafter “high-temperature alloys containing extremely reactive elements” or simply, “high-temperature alloys”). Depending upon the application, components made from such high-temperature alloys may possess sufficient high-temperature oxidation resistance and/or sulfidation resistance to be used bare without an applied protective coating. The high-temperature alloys are suited for long-term continuous exposure at temperatures as low as about 1300° F. and as high as about 2100° F. (1150° C.). The excellent oxidation protection and/or sulfidation protection afforded by these high-temperature alloys requires that the extremely reactive element, such as La or Y, be in the alloyed solution, and not as reaction products like oxides, carbides, nitrides, sulfides, etc. Unfortunately, these elements are extremely reactive to oxygen, carbon, nitrogen, sulfur, etc. and form such reaction products in the high-temperature alloy in which the extremely-reactive element(s) is contained, as well as in the components manufactured from the high-temperature alloy containing the one or more extremely-reactive elements. Alloys other than high-temperature alloys that include extremely reactive elements also benefit from maintaining the reactive element in alloyed form. For example, aluminum alloys having melting points starting about 1400° F. depend on La being able to freely dissolve without forming an oxide film or oxide inclusions that would interfere with that process. Therefore it is important to maintain the La in alloyed form in aluminum alloys.
Extremely reactive elements other than La and Y also provide beneficial properties if maintained in their alloyed form. For example, other extremely reactive elements in the lanthanide family of elements such as neodymium and samarium when alloyed with iron (Fe) beneficially form very strong “rare earth magnets”.
Conventional fabrication techniques for components with relatively complex three-dimensional (3D) geometries include forging, casting, and/or machining. Such conventional techniques are not only expensive and have long lead-times, but may additionally have low yields. Development time and cost for certain components may also be magnified because such components generally require several iterations. Moreover, a fundamental problem existing with conventional fabrication techniques is directly related to the extreme reactivity of elements (such as La and Y) to form very strong oxides or other compounds (reaction products) as described above.
For example, with respect to the extremely reactive element lanthanum (La), current practices for retention of a minimum beneficial content of alloyed La in a wrought form or as a casting rely on adding excess amounts of the extremely reactive element(s) to molten alloy to compensate for expected losses as oxides, etc. followed by quickly solidifying the alloy to retain the alloyed form of La, taking advantage of kinetics to manage the unfavorable thermodynamics. One downside risk with adding excessive levels of La is that localized regions of the molten alloy may be less exposed to oxidizing conditions. Thus, those regions may be overly enriched in La, possibly resulting in embrittlement or other metallurgical defects in the component. At the very least, significant variability in the concentration of alloyed La in the molten alloy may occur, depending on stirring, diffusion, reaction, etc. Because of the segregation of La in the alloyed form and the variability in formation of oxides, conventional methods of sampling to verify chemical composition may not be a sufficiently reliable indicator or predictor of having a desired controlled amount or level of alloyed La present where it is needed. Additive-manufactured components of high-temperature alloys containing the extremely reactive element La also fail to consistently achieve the optimal level of oxidation protection and/or sulfidation protection due to the depletion or chemical loss of a significant portion of the beneficial alloyed La to detrimental oxides. Similar issues exist with the other extremely reactive elements (i.e., those elements extremely reactive to forming oxides, carbides, nitrides, sulfides, etc. or other compounds based on thermodynamics as illustrated by Ellingham diagrams).
Hence, there is a need for methods for producing alloy forms from alloys containing one or more extremely reactive elements and methods for fabricating a component therefrom. Such methods enable the one or more extremely reactive elements in the alloy to be maintained in the beneficial alloyed (metallic) form at a controlled useful level, thereby conferring beneficial properties (e.g., oxidation-resistance, sulfidation-resistance and/or unique magnetic properties (at ambient temperature)) to the fabricated component. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.